The present invention relates to a semiconductor circuit device, and more specifically to a semiconductor circuit device whose substrate impedance is adjustable according to supply voltage fluctuations.
In semiconductor memory devices, a substrate bias voltage is often applied to a semiconductor substrate, in order to prevent a parasitic pn junction from being biased in the forward direction due to the undershoot of an external signal or to increase the circuit operation speed by increasing the depletion layer width of a junction for reduction of a parasitic capacitance.
FIG. 4B shows a so-called charge-pump circuit which can generate a substrate bias voltage. In this charge-pump circuit, when a pulsed input signal as shown in FIG. 4A is inputted to a node N40, an N-channel transistor TR2 pumps up electric charge from a semiconductor substrate and further accumulates it into a capacitor C2. Further, after an N-channel transistor TP1 has accumulated this accumulated charge into a capacitor C1, the N-channel transistor TR1 discharges it to a ground potential V.sub.SS, so that a substrate voltage V.sub.SUB can be outputted from a node N41.
FIG. 5 shows the substrate voltage characteristics generated by the charge-pump circuit as shown in FIG. 4B. In FIG. 5, when the supply voltage V.sub.CC decreases from the ordinary voltage V.sub.CC1 to another voltage V.sub.CC2, the substrate voltage V.sub.SUB changes from V.sub.SUB1 to V.sub.SUB2 in the negative direction. That is, when the supply voltage V.sub.CC drops abruptly from V.sub.CC1 to V.sub.CC2 as shown in FIG. 6, the substrate voltage once drops down to voltage V.sub.SUB1D lower than the voltage V.sub.SUB1 and then returns to the voltage V.sub.SUB2 into a stable condition after a time T represented by a time constant T=C.multidot.R has elapsed, where C denotes a substrate capacitance and R denotes a substrate impedance.
In this case, the relationship between the substrate voltage V.sub.SUB and the substrate current I.sub.SUB, that is, the load characteristics of the substrate bias voltage generating circuit can be represented as shown in FIG. 7, in which almost no current flows through the substrate when the substrate voltage changes from V.sub.SUB1D to V.sub.SUB2. Therefore, although the substrate impedance is substantially decided by only leakage current flowing through PN junctions formed in the substrate, since the leakage current is extremely small, the substrate impedance R is extremely high. Consequently, the time T required when the substrate voltage returns from V.sub.SUB1D to V.sub.SUB2 becomes long due to this high substrate impedance R. This causes the following problems:
When the supply voltage drops abruptly, since the substrate voltage V.sub.SUB once drops and then increases as shown in FIG. 8A, the threshold voltage V.sub.thn of each transistor formed on the same substrate changes as shown in FIG. 8B. This is caused by a back-gate bias effect such that the threshold voltage V.sub.thn increases with decreasing substrate voltage V.sub.SUB in the negative direction as shown in FIG. 9. Therefore, the limit voltage V.sub.CC-min at which each element formed on the substrate operates normally is largely dependent upon the threshold voltage V.sub.thn, as shown in FIG. 10. Accordingly, the limit voltage V.sub.cc-min changes when the threshold voltage V.sub.thn changes, and becomes stable when the threshold voltage V.sub.thn becomes stable, as shown in FIG. 8C.
In other words, when the supply voltage V.sub.CC fluctuates, a long time T required until the substrate voltage V.sub.SUB becomes stable, causes an unstable operation of the respective elements formed on the substrate. In particular, where data stored in memory devices are backed up by a battery and therefore the supply voltage drops momentarily, there exists a serious problem in that the data stored in the memory devices are not kept stored.
As described above, in the prior-art semiconductor circuit device, since it takes a long time until the substrate voltage becomes stable whenever the supply voltage fluctuates, there exists a problem in that the operation of the circuit formed on the substrate is unstable.